The synthesis of myofibrillar proteins and their mRNAs are being examined in quail myoblasts undergoing differentiation in cell culture. Amino acid incorporation studies have shown that subunits the synthesis of at least 8 contractile protein subunits of actin, myosin, tropomyosin and troponin is initiated coordinately when myoblasts fuse. The accumulation of mRNAs for all of these contractile proteins is initiated concident with this increase in contractile protein synthesis, as determined by translation of RNAs isolated from muscle cells in reticulocyte and wheat germ extracts. Differentiating muscle cells also synthesize and accumulate large amounts of long-lived mRNAs after fusion. These findings indicate that the activation contractile protein synthesis during myoblast differentiation is regulated by initiation of the transcription of the mRNAs for the contractile proteins and not by post-transcriptional regulation of mRNA translation. To directly examine the transcription of contractile protein mRNAs, cDNA copies of muscle mRNAs have been cloned using recombinant DNA techniques and these cloned DNA sequences are presently being characterized and identified.